1. Field of the Invention
This invention relates to a preform used for making a plastic can body without requiring a cutting process and to a method of manufacturing a plastic can body by using such a preform.
2. Description of the Related Art
Fluids, such as beverages and oils, are conventionally contained in iron or aluminum cans. Along with the wide use of plastic bottles in place of glass bottles, there is a large demand for can-like plastic containers that can hold fluids.
Examples of such containers are disclosed in Japanese Patent Publication Sho-58 501991, and Japanese Patent Laid-Open Publications Sho-60 25734 and Sho-60 72720.
With these conventional containers, a preform is made by an injection molding process. The preform is then subjected to a biaxial stretch blow molding process to form a container having a flange around its top. The container is cut at an unnecessary portion of the flange by a cutter. The flange serves to hold a lid to the container by forming a double seam.
The blow-stretched flange is small in thickness but has sufficient mechanical strength due to the orientation of the plastic material. The flange also has properties to prevent permeation of gas into the container. Since such a blow-stretched flange is formed on part of the side wall of the blow-stretched container, it is necessary to cut an .unnecessary portion of the flange by a cutter or the like. Consequently, the cut edge of the flange may be nicked. Even if the flange is formed by the biaxial stretch blow molding process to have the desired stretching orientation, extensive stresses will be applied to the cut edge. This can cause cracking of the cut edge and reduce the mechanical strength and durability when the flange is curled and double-seamed with the lid.
Forming the flange by the stretch blow molding process largely depends upon factors such as the selected material and the molding process. Therefore, it is very difficult to produce a flange having a uniform thickness. The flange may have different thicknesses along its circumferential direction.
In the foregoing methods, the cutting process not only complicates the container manufacturing process but also makes the flange less dimensionally uniform and durable.